General Motors announced Tuesday that its forthcoming electric vehicle, the Chevrolet Volt, will get fuel economy of 230 miles per gallon in city driving, an achievement that both accelerates and befogs the industry's race to produce more efficient cars.

The Volt will become the first mass-produced vehicle to obtain a triple-digit mpg rating, the company said, and some industry analysts agreed that the car may put the company a step ahead of competitors building plug-in hybrids.

"The GM vehicle is the best one in the country based on what we've seen," said Don Hillebrand, director of transportation research at the Energy Department's Argonne National Laboratory, the nation's leading lab for plug-in vehicles. "But it's hard to tell -- other companies could be keeping theirs secret."

Indeed, most of the major automakers are clamoring to become known as the "greenest" of all. GM has already begun an advertising campaign for the Volt based around a logo that forms a "230" using a 2, a 3 and a round electrical outlet. Company officials hope the car, scheduled for late next year, will come to symbolize the "new GM" formed after bankruptcy and a federal bailout.

But the company's focus on a miles-per-gallon standard also highlights the ongoing confusion over how to measure the energy efficiency of cars that run on both gas and electricity.

The conventional mpg standard doesn't easily fit electric cars and has suddenly become almost infinitely elastic.

Earlier this month, Nissan announced a new all-electric vehicle known as the Leaf. And while it won't take a drop of gas, the company said it gets the equivalent of 367 mpg, citing an Energy Department conversion of electricity to gasoline. ("Nissan Leaf = 367 mpg, no tailpipe, and no gas required. Oh yeah, and it'll be affordable too!" the company said on its Twitter feed).

Similarly, manufacturers of equipment that allows Toyota Priuses to be recharged through a plug have declared that their hardware achieves the equivalent of more than 100 miles per gallon, though those claims have been disputed.

As for GM, its claim of 230 miles per gallon is based on a proposed federal methodology for measuring the efficiency of plug-ins. But what Volt drivers will actually get for mileage depends on how far and how hard they drive.

The first 40 miles in a fully charged Volt is largely powered by the battery, company officials said. The car can extend its range to more than 300 miles with its flex fuel-powered engine-generator.

So if, for example, a person drives a Volt only 30 miles a day on easy roads and recharges each night, almost no gasoline would be required, making its mileage almost infinite, company officials said.

But the 230 mpg claim does not mean that a person could drive the equivalent of that distance -- say, from the District to Pittsburgh -- on a single gallon of gas. The car would need to be recharged multiple times along the way.